1. Field of the Invention
This invention relates to electronic engine controls.
2. Prior Art
It is known to use an electronic engine control module to control the amount of fuel being injected into an engine. In particular, it is known to use the output of an exhaust gas oxygen sensor as part of a feedback control loop to control air/fuel ratio. Typically, such an exhaust gas oxygen sensor is placed upstream of the catalyst which processes the exhaust gases. In some applications it is known to use a second exhaust gas oxygen sensor downstream of the catalyst, partly to serve as a diagnostic measurement of catalyst performance. With the presence of exhaust gas oxygen sensors both upstream of the catalyst and downstream of the catalyst, it would be desirable to develop an improved feedback air/fuel ratio control system using signals from both of these sensors.
Referring to FIG. 1, a prior art A/F control system 10 for an engine 11 uses feedback from an exhaust gas oxygen (EGO) sensor 12 installed after a catalyst 13 to trim the control point of a pre-catalyst A/F feedback loop including a pre-catalyst EGO sensor 14, a pre-catalyst feedback controller 15 and a base fuel controller 16. This post-catalyst feedback aids in (1) compensating for aging of pre-catalyst EGO sensor 14, and (2) maintaining the engine A/F in the catalyst window. Such performance improvements help reduce vehicle exhaust emissions. In known system designs, feedback from the post-catalyst sensor is used to slowly trim the A/F of the pre-catalyst loop by either changing the set point of the pre-catalyst EGO sensor or changing the relative values of the up-down integration rates and/or jump back values in the pre-catalyst control loop. A post-catalyst feedback loop includes a post-catalyst feedback controller 17 coupled between post-catalyst EGO sensor 12 and pre-catalyst feedback controller 15.
However, in such post-catalyst/pre-catalyst feedback systems (1) the pre-catalyst EGO sensor exhibits A/F offset errors which vary as a function of engine rpm and torque, and (2) the post-catalyst EGO sensor feedback signal is delayed due to oxygen storage in the catalyst. Since engine rpm and torque change continuously during dynamic operating conditions, the A/F correction applied to the pre-catalyst feedback loop under these conditions may not occur at the same rpm/torque point which generated the feedback signal, and the A/F offset error will consequently be incorrectly trimmed. As a result, such post-catalyst/pre-catalyst feedback systems compensate for aging of the pre-catalyst EGO sensor on the average basis. They do not maintain the engine A/F in the catalyst window at all rpm/torque operating points of the enqine. It would be desirable to have a system to not only compensate for pre-catalyst EGO sensor aging, but to also maintain the engine A/F in the catalyst window for all rpm/torque operating conditions.